Narcolepsy is a chronic neurological disorder caused by the brain's inability to regulate sleep-wake cycles normally. At various times throughout the day, people with narcolepsy experience fleeting urges to sleep. If the urge becomes overwhelming, patients fall asleep for periods lasting from a few seconds to several minutes. In rare cases, some people may remain asleep for an hour or longer.
Narcoleptic sleep episodes can occur at any time, often without warning, and may be quite dangerous if patients are driving or operating machinery. In addition to daytime sleepiness, patients may experience cataplexy, or the sudden loss of voluntary muscle tone; vivid hallucinations during sleep onset or upon awakening; brief episodes of total paralysis at the beginning or end of sleep; and/or automatic behavior, such as talking or performing routine activities during a sleep episode but having no memory of these activities upon waking. Most patients also experience frequent awakenings during nighttime sleep. For these reasons, narcolepsy is considered to be a disorder of the normal boundaries between the sleeping and waking states.
Eleven families of phosphodiesterases (PDEs) have been identified but only PDEs in Family I, the Ca2+-calmodulin-dependent phosphodiesterases (CaM-PDEs), have been shown to mediate the calcium and cyclic nucleotide (e.g. cAMP and cGMP) signaling pathways. The three known CaM-PDE genes, PDE1A, PDE1B, and PDE1C, are all expressed in central nervous system tissue. PDE1A is expressed throughout the brain with higher levels of expression in the CA1 to CA3 layers of the hippocampus and cerebellum and at a low level in the striatum. PDE1A is also expressed in the lung and heart. PDE1B is predominately expressed in the striatum, dentate gyms, olfactory tract and cerebellum, and its expression correlates with brain regions having high levels of dopaminergic innervation. Although PDE1B is primarily expressed in the central nervous system, it may be detected in the heart. PDE1C is primarily expressed in olfactory epithelium, cerebellar granule cells, and striatum. PDE1C is also expressed in the heart and vascular smooth muscle.
Cyclic nucleotide phosphodiesterases downregulate intracellular cAMP and cGMP signaling by hydrolyzing these cyclic nucleotides to their respective inactive 5′-monophosphates (5′AMP and 5′GMP). CaM-PDEs play a critical role in mediating signal transduction in brain cells, particularly within an area of the brain known as the basal ganglia or striatum. For example, NMDA-type glutamate receptor activation and/or dopamine D2 receptor activation result in increased intracellular calcium concentrations, leading to activation of effectors such as calmodulin-dependent kinase (CaMKII) and calcineurin and to activation of CaM-PDEs, resulting in reduced cAMP and cGMP. Dopamine D1 receptor activation, on the other hand, leads to activation of calcium dependent nucleotide cyclases, resulting in increased cAMP and cGMP. These cyclic nucleotides in turn activate protein kinase A (PKA; cAMP-dependent protein kinase) and/or protein kinase G (PKG; cGMP-dependent protein kinase) that phosphorylate downstream signal transduction pathway elements such as DARPP-32 (dopamine and cAMP-regulated phosphoprotein) and cAMP responsive element binding protein (CREB).
CaM-PDEs can therefore affect dopamine-regulated and other intracellular signaling pathways in the basal ganglia (striatum), including but not limited to nitric oxide, noradrenergic, neurotensin, CCK, VIP, serotonin, glutamate (e.g., NMDA receptor, AMPA receptor), GABA, acetylcholine, adenosine (e.g., A2A receptor), cannabinoid receptor, natriuretic peptide (e.g., ANP, BNP, CNP) and endorphin intracellular signaling pathways.
Phosphodiesterase (PDE) activity, in particular, phosphodiesterase 1 (PDE1) activity, functions in brain tissue as a regulator of locomotor activity and learning and memory. PDE1 is a therapeutic target for regulation of intracellular signaling pathways, preferably in the nervous system, including but not limited to a dopamine D1 receptor, dopamine D2 receptor, nitric oxide, noradrenergic, neurotensin, CCK, VIP, serotonin, glutamate (e.g., NMDA receptor, AMPA receptor), GABA, acetylcholine, adenosine (e.g., A2A receptor), cannabinoid receptor, natriuretic peptide (e.g., ANP, BNP, CNP) or endorphin intracellular signaling pathway. For example, inhibition of PDE1B may potentiate the effect of a dopamine D1 agonist by protecting cGMP and cAMP from degradation, and similarly inhibit dopamine D2 receptor signaling pathways, by inhibiting PDE1 activity. PDE1 inhibitors are therefore potentially useful in diseases characterized by reduced dopamine D1 receptor signaling activity. See generally, WO 03/020702.
EP 0201188 and EP 0911333, the contents of which are incorporated herein by reference, disclose certain 1,3,5,-substituted, 6,7-dihydro-1H-pyrazolo[4,3-d]pyrimidin-7-one compounds, claimed to be useful for treatment of cardiovascular disease, erectile dysfunction, and other disorders. These compounds are not, however, taught or suggested to be useful for the treatment of diseases involving disorders of the dopamine D1 receptor intracellular pathway, particularly diseases relating to sleep disorders such as narcolepsy. PCT/US2006/33179, the contents of which are incorporated herein by reference, discloses the use of 1,3,5,-substituted, 6,7-dihydro-1H-pyrazolo[4,3-d]pyrimidin-7-one compounds for treatment of diseases involving disorders of the dopamine D1 receptor intracellular pathway, but does not specifically disclose the use of such compounds in the treatment or prophylaxis of narcolepsy. PCT/US2006/022066, the contents of which are incorporated herein by reference, discloses PDE1 inhibitors which are 7,8-dihydro-[1H or 2H]-imidazo[1,2-a]pyrazolo[4,3-e]pyrimidin-4(5H)-ones or 7,8,9-trihydro-[1H or 2H]-pyrimido[1,2-a]pyrazolo[4,3-e]pyrimidin-4(5H)-ones, but does not specifically disclose their use for the treatment or prophylaxis of narcolepsy. WO 03/042216, U.S. Pat. No. 5,939,419, EP 0 538 332, U.S. Pat. No. 5,393,755, U.S. Pat. No. 6,969,719 B2, Xia et al., J. Med. Chem. (1997), 40, 4372-4377 and Ahn et al., J. Med. Chem. (1997), 40, 2196-2210, the contents of all of which are incorporated herein by reference, disclose PDE1 cGMP phosphodiesterase inhibitors which are substituted pyrazolo[3,4-d]pyrimidine, pyrimido[2,1-b]purin-4-one and imidazo[2,1-b]purin-4-one analogues useful for the treatment of hypertensive, cardiovascular, sexual dysfunction and other cGMP-PDEV related disorders, but do not specifically disclose their use for the treatment or prophylaxis of narcolepsy.